1. Field of the Invention
Embodiments of the present invention generally relate to the field of semiconductor manufacturing and more specifically to a method and apparatus for the selective oxidation of a composite silicon/metal film.
2. Description of the Related Art
In the manufacture of semiconductor devices, oxidation of silicon containing substrates plays a key role. For example, in a standard semiconductor device, a gate oxide layer is ordinarily situated over a substrate containing a source region, a drain region, and an intervening silicon or polysilicon region. Metal contacts are deposited over the source and drain regions, and a conductive layer deposited over the gate oxide. The entire structure is often depicted as a stack of layers. When a voltage is applied across the gate oxide generating an electric field oriented along an axis from the substrate, through the gate oxide, to the conductive layer, the electrical characteristics of the region between the source and drain region change, either allowing or stopping the flow of electrons between the regions. The gate oxide layer thus occupies a pivotal role in the structure of semiconductor devices.
Often, properties of the device are improved by deposition of other layers in the device. For example, to control diffusion of metal atoms into the gate oxide layer, which degrades the dielectric properties of the gate oxide, a barrier layer may be deposited between the gate oxide and the metal layer. Also, a hard mask layer may be deposited over the metal layer. In order to promote adhesion of such layers, smooth their surfaces, and harden them to diffusion, the barrier or hard mask layers may be treated with a plasma. The plasma treatment can degrade the properties of the gate oxide layer by eroding it from the sides or reducing its thickness. Likewise, the gate oxide layer may be damaged by repeated cycles of deposition, etching, and plasma processing typically involved in modern device fabrication. This damage degrades the gate characteristics of the layer, rendering the device inoperative.
To repair the damage to the oxide layer, it is possible to re-oxidize the device. Re-oxidation creates a thin layer of oxide on the sides of the gate oxide and underlying silicon containing layers, repairing the edge damage. Because oxidizing other regions of the transistor may reduce conductivity and impair the device, oxidizing only certain materials in the device is desired. For example, oxidizing the metal cap over the gate and the metal contacts over the source and drain regions reduces their conductivity. Likewise, a given device may contain more than just the metal surfaces associated with transistors. Selective oxidation targets certain materials, such as silicon and oxides of silicon, while avoiding oxidation of other materials.
Conventional oxygen rich processes oxidize not only the desired layers, but also undesired layers such as metals and barrier layers. Wet oxidation processes, although faster than dry processes, do not promote oxide growth as quickly as steam oxidation. FIGS. 1A-1C set forth oxidation rates of silicon for dry oxidation, wet oxidation, and steam oxidation, respectively. Subjecting a device to heat under an atmosphere of dilute steam rich in hydrogen gas (H2) at low pressure can selectively oxidize silicon containing materials without oxidizing metals or barrier layers. As can readily be appreciated, however, operating a hydrogen combustion chamber at high temperature and pressure has heretofore required combustion of hydrogen in a separate location. At higher pressures, and with long soak times, hydrogen gas may attack barrier and hard mask layers, reducing their effectiveness and forming unwanted metal silicide layers with higher resistivity.
Thus, there is still a need for a selective oxidation process utilizing in situ steam generation that efficiently oxidizes only silicon containing layers of a semiconductor device stack without degrading the properties of barrier or conductive layers.